This invention relates to improvements to free-piston internal combustion engines, especially those which are intended to be used for the operation of electric generators with linear alternators.
It is known that the internal combustion engines with connecting rod and crank, especially when used for the operation of electric generators. are unsatisfactory as regards both their low efficiency and the production of harmful gases to which they give rise. Their substitution by gas turbines is only possible in case of very high powers, on account of the high costs of such turbines and the technological problems these turbines cause. For middle power applications the so-called free-piston engines have been proposed. In these engines, owing to the absence of linkage, it is essentially the inertia of the movable parts which delays the displacement of the piston immediately after the beginning of the combustion, thereby ensuring the availability of a sufficient time for the vaporization, mixing and combustion in the chamber the cylinder, whilst the thrust corresponding to the high pressures which are generated during this stroke, instead of being hindered by linkage, is accumulated in the movable parts in the form of kinetic energy and is utilized in the continuation of the cycle. Therefore, in these engines, in addition to the constructional simplifications and the reduction of the mechanical losses, directly inherent to the elimination of linkage, conditions are created which are more favourable for the conversion of the energy, with consequent increase of the efficiency and reduction of the thermal losses, the temperature of the exhaust gas and the content, in these latter, of pollutant substances, especially nitric oxide.
However, the free-piston engines in their turn cause serious problems which have not yet been completely solved; among these problems, the most serious regard the supply of energy for carrying out the compression stroke, the synchronization of the movement of the various pistons and the limitation of the travel of these latter, which problems, in the conventional engines, are solved by the presence of linkage connected to a driving shaft provided with a flywheel, whilst no corresponding parts are provided in the free-piston engines.
For the execution of the compression stroke it has been proposed (Jarret) to provide hydraulic springs counteracting the pistons, which springs however involve constructional difficulties and efficiency losses and give rise to losses of liquid; the particular problem is solved, during normal operation (Allais), by an arrangement of opposed cylinders, but in the case of lack of combustion this solution becomes ineffective if it is not integrated by other means, which hitherto were formed by gearing or linkage which it would be preferable to avoid. No effective solution which does not involve the use of linkage or gearing has been provided so far to ensure the limitation of the travel of the pistons to a predetermined value, or to ensure the synchronism in the movement of the pistons in an engine comprising a plurality of units.